This invention relates to polyurethane polymers, particularly air curable water-borne urethane-acrylic hybrid polymers, and more particularly, air curable water borne urethane-acrylic hybrid polymers suitable for coatings.
It is known in the coating industry that polyurethane coatings exhibit excellent resistance to abrasion, chemicals and solvents, are highly flexible and durable. Conventional oil modified urethanes prepared in organic solvents such as aliphatic hydrocarbons are applied as clear or pigmented coatings. Once applied, the solvent evaporates and the film is crosslinked by air oxidation through the ethylenic unsaturation in the oil. A major application for these coatings has been as clear coatings for wood flooring. Higher solids versions of these coatings are less polluting than their conventional counterparts that often have high VOC levels; however, there is some compromise in performance, particularly dry rate and hardness.
An alternative to limit VOC's, yet maintain performance, is to use water dispersible polyurethanes. For example, water-borne anionically stabilized polyurethanes are made by the reaction of polyalcohols and dihydroxy carboxylic acid with an excess of diisocyanate to produce an NCO terminated carboxy functional prepolymer. This prepolymer is neutralized with a tertiary amine to form salt groups and dispersed into water. The terminal NCO groups are then reacted with active hydrogen containing compounds having functionalities of two or more and reactivities greater than that of water to produce a fully reacted polyurethane polymer. To facilitate manufacture, the prepolymer is typically made in the presence of a solvent that is either left in the dispersion, or removed as one of the last steps in production to provide a solvent-free product. If the solvent is left in the dispersion the typical solvent used is N-methyl pyrrolidinone. If the solvent is to be removed, a more volatile solvent is employed such as acetone or methyl ethyl ketone. As alternatives to the preferred anionic stabilizing group, water dispersible polyurethane polymers may be prepared containing cationic stabilizing groups or non-ionic stabilizing groups to facilitate water dispersibility. For the most part these polymers are linear and their films vary from hard and relatively inflexible to soft and highly flexible. The applications for these water-borne urethane polymers include clear and pigmented coatings for concrete, metal, wood, semi-rigid and flexible plastics, rubber and leather; glass fiber sizing, printing inks and adhesives.
U.S. Pat. Nos. 4,066,591 and 4,147,679, to Scriven et al., propose that water-borne polyurethanes can be prepared from NCO terminated urethane prepolymers containing air curable ethylenic unsaturation and carboxylic functionality. Once dispersed, these prepolymers are chain extended with polyamines, hydrazine, hydrazides or mixtures thereof. While these dispersions do contain some air curable ethylenic unsaturation they are still ensentially linear and even when crosslinked by air oxidation of the ethylenic unsaturation they are unable to provide the necessary mar, scuff and chemical resistance for applications such as coatings for wood flooring.
As previously mentioned, water-borne polyurethane polymers are for the most part linear, producing films with poorer chemical resistance than the highly crosslinked films of two component solvent-borne urethane systems. Pre-crosslinking of the water-borne polyurethane polymers by incorporation of a significant amount of monomer(s) having functionalities greater than two in the prepolymer stage results in highly viscous prepolymers that can not be dispersed. Sufficient pre-crosslinking of the prepolymer in the dispersion stage by means of chain extenders having functionalities greater than two can result in significant polymer gel. For improved chemical resistance, anionically stabilized water-borne polyurethane polymers are typically formulated with a second component to effect post crosslinking. The term "post crosslinking" refers to a chemical reaction designed to occur during and after application of the film. Typical crosslinking agents for anionically stabilized water-borne urethanes include polyaziridine, carbodiimide and epoxies. Once the volatile neutralizing amine has evaporated from the film the polyaziridine reacts with the acid group on the urethane polymer backbone. This reaction occurs at amibient temperature while in the case of carbodiimide and epoxy crosslinking, elevated temperatures are necessary to effect complete cure. A major deficiency of these crosslinking technologies is the limited pot-life. Dispersions containing sufficient residual hydroxyl groups may be formulated in conjunction with melamine into single package systems that are stable at room temperature, however, elevated temperatures are required to effect crosslinking.
Blending of acrylic emulsions with water-borne polyurethanes has been used as a means to reduce coating costs, however, there is some compromise in performance such as chemical resistance. U.S. Pat. No. 4,644,030 to Loewrigkeit et al. proposes that non-self crosslinkable, essentially emulsifier free aqueous polyurethane dispersions can be prepared by producing an NCO terminated carboxylic acid functional prepolymer in the presence of inert liquid vinyl monomer(s). The carboxylic acid group of the prepolymer is neutralized with a volatile amine and the blend dispersed into water. This neutralized, dispersed NCO terminated prepolymer is then chain extended with one or more active hydrogen containing compounds such as polyamines, hydrazine and hydrazides. This dispersion consisting of polyurethane polymer and vinyl monomer(s) is then subjected to free radical polymerization. While dispersions of this type may be essentially free of any cosolvent and emulsifier and lower in raw material costs compared to pure polyurethane dispersions, the polymer is essentially linear and, thus, does not provide sufficient chemical resistance in the absence of any second component crosslinking agents mentioned previously.
U.S. Pat. No. 5,571,857 to Gruber et al. proposes that solvent free urethane/acrylic hybrid polymers can be made by reaction of an excess of isophorone diisocyanate with polyol(s) and dihydroxy carboxylic acid. The resulting NCO terminated carboxylic acid functional prepolymer is blended with vinyl monomer(s) to reduce the viscosity. A tertiary amine is then added to neutralize the acid groups and the blend dispersed into water. The NCO terminated urethane prepolymer is then chain extended with a blend of mono and diamines followed by free radical polymerization of the vinyl monomer(s). As with U.S. Pat. No. 4,644,030, to Loewrigkeit these polymers are essentially linear and, chain termination is due to the use of mono-amine limits the molecular weight. These polymers are solvent and emulsifier free, however, in the absence of a second component, the chemical resistance is insufficient for coatings applications.
It is proposed in U.S. Pat. No. 5,521,246 to Tien et al. that a room temperature self-crosslinkable aqueous dispersion can be prepared by first producing an NCO-terminated carboxyl functional polyurethane prepolymer, adding vinyl monomers a portion of which is glycidyl methacrylate, neutralizing the carboxyl groups with a tertiary amine, dispersing the mixture into water, adding an oil-soluble free radical initiator, polymerizing the vinyl monomers and chain extending the urethane with water. Since the epoxy group is hydrophilic and reaction catalyzed by the presence of tertiary amines, the stability of these dispersions is questionable particularly at elevated temperatures.
Ketone-hydrazide crosslinking technology involving two distinct polymers is disclosed in U.S. Pat. No. 5,141,983 to Hasegawa et al. This type system consists of an acrylic polymer containing ketone functionality and a urethane polymer that contains hydrazide terminal groups or amido groups. After mixing, application and removal of water and neutralizing agent, a chemical bond is formed between these two distinct polymers resulting in a film with improved solvent resistance over simple blends. The removal of water from these films is highly dependent on the temperature and humidity. Thus, to ensure complete ketone-hydrazide reaction, an elevated temperature is generally required. While providing some improvement in performance, incorporation of both air curable ethylenic unsaturation and ketone-hydrazide crosslinking technologies into a single coating system offers a higher degree of crosslinking and this crosslinking is not as dependant on complete removal of water.